Automated laboratory systems in which weighing processes are performed with the help of robots belong to the known state of the art. Such systems are normally used in situations where weighing operations are performed serially in large numbers, where the automation brings labor cost savings and improved reliability.
A typical example for an automated serial weighing process occurs in the weighing of filters that are used in air pollution tests, for example in the testing of diesel engines. In these tests, the exhaust gases that are to be tested are sent through a filter which holds back solid particles, in particular soot particles. The filters which are individually marked and traceable are weighed a first time in their new, unused condition, and the initial weight of each filter is registered in a database. The filters are then used for collecting the particles in the equipment under test and are subsequently weighed again. Next, the net weight of the combustion residues collected by the filter is determined by subtracting the initial weight from the end weight.
A system for the weighing of filters with the help of robots is described for example in U.S. Pat. No. 5,606,153, wherein the flat, circular filters are seated in ring-shaped holders that carry a barcode identification. Arranged on a vibration-isolated weighing table are a swivel-arm robot, a microbalance, an electrostatic discharging device, a carousel tray holding the filters, as well as a device for temporarily parking the ring-shaped holder of the filter that is in the process of being weighed. To carry out a weighing operation, the robot first transports a filter that is seated in a ring-shaped holder from the carousel tray to the temporary parking station, where the filter is separated from the ring-shaped holder device. The robot then moves the filter without the ring-shaped holder through the electrostatic discharging device to the microbalance, where the functions of the balance—opening and closing the door, setting the balance to zero, recording the weighing result and transmitting it to a computer—run automatically and are coordinated with the movements of the robot. The filter is returned to the temporary parking device and inserted into the ring-shaped holder, whereupon the ring-shaped holder with the filter is returned to the carousel tray.
In the robotic weighing system of the foregoing description, a commercially available microbalance is used in which the aforementioned, normally manual functions can also be executed automatically, i.e. in response to control commands of a computer. However, with this kind of balance, one has to accept that a balance that is designed to satisfy the ergonomic requirements of manual operation and of a wide range of applications will in some respects not be optimally tailored to the needs of automated filter weighing. In particular, a draft shield enclosure in the standard version of a commercially available microbalance has a taller interior space than is necessary for filter weighing. With a lower profile of the draft shield enclosure, the air turbulence associated with the opening and closing of the weighing compartment door could be reduced and the settling time of transient oscillations of the balance could be shortened. In addition, if the balance is operated automatically, the draft shield enclosure does not need to be transparent and can therefore be made of metal, whereby the problem of electrostatic charges is eliminated.
A filter-weighing system which was developed by the applicant and which is being distributed in Germany by the firm Horiba under the name PWS ONEplus™ includes an XYZ-robot, a microbalance, a rack for holding the filters with several shelves arranged vertically above each other, as well as a computer to control the system and to process and store the data. The flat, circular-shaped filters are individually contained in suitably shaped receptacles which carry a barcode identification, whereby the filter that is currently held by the receptacle is individually identified. The bottoms of the receptacles have a circular opening whose diameter is smaller than the filter diameter, but larger than the weighing-pan diameter of the microbalance. To weigh a filter, the receptacle is moved to a centered position over the weighing pan and lowered onto the floor of the weighing compartment, whereby the filter is transferred to the weighing pan and lifted off the receptacle bottom. Consequently, the filter does not have to be taken out of the receptacle for the weighing.
The microbalance in the filter-weighing system just described is a serial-production model manufactured by the applicant. It has a transparent, cylindrical draft-protection enclosure made of glass, with a cylindrically curved sliding door that opens and closes in a swivel movement about the cylinder axis, driven by a motor that is controlled by command signals from the computer. Due to the concept of the filter-weighing receptacles, the problem of electrostatic charge accumulation on the filter is avoided with this filter, but as in the earlier example, the standard-production draft-protection enclosure is taller than would be necessary for filter-weighing.
To meet the objections against the use of a standard-production draft shield enclosure, the applicant's first approach was to develop a low-profile draft shield enclosure that was made of metal and tailored specifically to work with the filter-weighing receptacles, but keeping the electric motor-driven door of the standard-production version. It was found, however, that the control of the door movement cannot be coordinated rigidly enough with the movement flow of the robot and that, as a consequence, the draft shield compartment door occasionally opens too late or not at all, causing the robot arm to collide with the closed draft shield door, whereby the filter-weighing system can become damaged.
The present invention therefore has the objective of providing a door-opening device for a balance draft shield enclosure that is optimally matched to the conditions imposed by a robotically operated filter-weighing system and which, in comparison to the existing state of the art, is distinguished by a simple, cost-effective design and by its functional reliability. In view of the robot being available for use, the motor drive and electronic control that are used in the standard version can be dispensed with, and the robot can also be put to work for the operation of the door-opening device.